The present invention relates to an antenna system of the diversity reception type, used mainly for a mobile wireless machine.
During recent years, a mobile radio communication market has expanded. With this, market needs for mobile wireless machines, in particular small and high performance mobile wireless machines, such as handyphones and PHS (personal handyphone system), are rapidly increasing. A PDC (personal digital cellular) system having frequency bands of 800 MHz and 1.5 GHz, is widely used in the current handyphone system in Japan. A whip antenna, retractable into the machine body, and another antenna, built in the machine body and used for diversity reception, are assembled into a handyphone, and one of these antennas is selected when the handyphone is operated for use.
FIG. 8 schematically shows a parts-layout of an antenna system in a conventional small wireless machine. In the figure, reference numeral 81 is a whip antenna and 82 is a built-in antenna. The whip antenna 81 is used as a transmission/reception antenna retractable into a radio machine body 83. The built-in antenna 82 is a sub-antenna for diversity reception, used only for the reception purpose. Of the two antennas, one antenna put in a higher receiving electric field than the other is selected by a select switch of the machine body, whereby to effect a diversity reception.
FIG. 9 is a circuit diagram showing a conventional antenna system of a small wireless machine. In the figure, numeral 91 is a whip antenna and 92 is a built-in antenna. Numeral 93 is an antenna matching circuit connected to the whip antenna 91, and 94 is an antenna matching circuit of the built-in antenna 92. The antenna matching circuits 93 and 94 are respectively connected at their nodes 95 and 96 to feeders 97 and 98, which are then connected to an antenna select switch 99. Of the two antennas 91 and 92, the antenna put in a higher receiving electric field than the other is selected by the antenna select switch 99, and connected to a radio circuit 100, to thereby realize a quality and stable mobile communication.
With a size reduction of the mobile wireless machine, the antennas is also reduced in size and further the two antennas are disposed more closely. Where the antennas are closely disposed, the whip antenna and/or the built-in antenna are reduced in their receiving sensitivity, so that a communication quality is degraded. Particularly in the diversity system shown in FIG. 9 in which only the antennas are switched one to the other for diversity reception, when the built-in antenna 92 is selected, the terminal condition of the whip antenna 91 is open in the antenna select switch 99 since it is not connected to the radio circuit 100. Such does not take place in a post-detection diversity system in which receiving circuits are provided in association with a main antenna and a sub-antenna, respectively. Actually, the feeder 97, for example, exists between the whip antenna 91 and the antenna select switch 99, so that the whip antenna and the select switch are spaced from each other a given distance. An impedance looking toward the radio circuit 100 from the node 95 of the antenna matching circuit 93 is zero (0.OMEGA.) or the transmission line extending from the node to the radio circuit is open at the end when an electrical distance of the transmission line is 1/4 wavelength. When the electrical length of the transmission line is 1/2 wavelength, the transmission line is open at the end or the impedance of the transmission line is infinity (.infin..OMEGA.). When the electrical distance of the transmission line is between 1/4 to 1/2 wavelength, the impedance of the transmission line is between 0.OMEGA. to .infin..OMEGA.. Thus, the impedance varies when the distance of the transmission line varies, or it is instable. The coupling of the whip antenna 91 and the built-in antenna 92 depends largely on the mutual terminal conditions of the those antennas. This results in necessity of controlling the mutual terminal conditions of the antennas.